1. Technical Field
The present invention relates to an atomic oscillator.
2. Related Art
An atomic oscillator based on an electromagnetically induced transparency (EIT) system (also called a coherent population trapping (CPT) system) is an oscillator using a phenomenon in which when two resonant lights different from each other in wavelength (frequency) are simultaneously irradiated to an alkali metal atom, the absorption of the two resonant lights is stopped.
It is known that the interaction mechanism between the alkali metal atom and the two resonant lights can be explained in a Λ-type three-level system model as shown in FIG. 13A. The alkali metal atom has two ground levels, and when resonant light 1 having a wavelength (frequency f1) corresponding to an energy difference between the ground level 1 and the excited level or resonant light 2 having a wavelength (frequency f2) corresponding to an energy difference between the ground level 2 and the excited level is individually irradiated to the alkali metal atom, light absorption occurs as is well known. However, as shown in FIG. 13B, when the resonant light 1 and the resonant light 2 in which the frequency difference f1−f2 accurately coincides with the frequency corresponding to the energy difference ΔE12 between the ground level 1 and the ground level 2 are simultaneously irradiated to the alkali metal atom, a superposition state of the two ground levels, that is, a quantum interference state occurs, the excitation to the excited level is stopped, and the transparency phenomenon (EIT phenomenon) occurs in which the resonant light 1 and the resonant light 2 pass through the alkali metal atom. This EIT phenomenon is used and an oscillator with high accuracy can be formed by detecting and controlling the abrupt change of light absorption behavior when the frequency difference f1−f2 between the resonant light 1 and the resonant light 2 shifts from the frequency corresponding to the energy difference ΔE12 between the ground level 1 and the ground level 2.
FIG. 14 is a schematic view of a general structure of a related art atomic oscillator of a CPT system. As shown in FIG. 14, the related art atomic oscillator of the CPT system modulates a drive current of a frequency f0 (=v/λ0: v is light speed, λ0 is the center wavelength of laser light) generated by a current drive circuit with a modulation frequency fm1 of ½ of a frequency corresponding to an energy difference ΔE12 between the ground level 1 and the ground level 2, so that a semiconductor laser generates a resonant light 1 having a frequency f1=f0+fm1 and a resonant light 2 having a frequency f2=f0−fm1 (FIG. 13B), and a gaseous alkali metal atom contained in an atomic cell causes the EIT phenomenon. In the atomic oscillator, the oscillation frequency of a voltage controlled crystal oscillator (VCXO) is controlled so that the detection amount of light passing through the atomic cell becomes maximum. A phase locked loop (PLL) multiplies the oscillation frequency by a multiplication ratio N/R (both N and R are positive integers) to generate a signal of the modulation frequency fm1 of ½ of the frequency corresponding to ΔE12. According to the structure as stated above, since the voltage controlled crystal oscillator (VCXO) continues the oscillation operation very stably, the oscillation signal with very high frequency stability can be generated. Incidentally, the atomic cell here includes the gaseous alkali metal atom and the container containing this, and its description will be made below.
U.S. Pat. No. 6,320,472 is an example of related art.
However, in the related art atomic oscillator, the oscillator, such as the PLL, to generate the modulation frequency fm1, which accurately coincides with the frequency of ½ of the frequency (for example, 9.19263 . . . GHz in the case of a cesium atom) corresponding to ΔE12, directly or by harmonics must be prepared specially, and the degree of freedom of design is restricted.
Besides, since stability is attained in the state where the modulation frequency fm1 accurately coincides with the frequency of ½ of the frequency corresponding to ΔE12, in the related art atomic oscillator, according to the accuracy of the multiplication ratio N/R of the PLL, there is a case where the voltage controlled crystal oscillator (VCXO) can not be accurately oscillated at the desired frequency (nominal frequency). Thus, for example, when the oscillation frequency of the voltage controlled crystal oscillator (VCXO) is 9.999 MHz, a high accuracy frequency conversion circuit is required to obtain merely 10 MHz of the nominal frequency, and there is a case where the atomic oscillator can not be realized by a simple structure (see FIG. 14).